1. Field of the Invention
This invention relates to protection circuits operating in conjunction with normally stable power supplies for providing power to a system of high intensity, gaseous discharge lamps and therefore susceptible to injury from a supply voltage exceeding even a momentary voltage level beyond a certain limit, and sensing even an overload of one polarity of ac voltage.
2. Description of the Prior Art
There are many fuses, circuit breakers and the like that operate in the presence of a voltage overload to open an incoming supply source to prevent injuring a delicate electrical device and/or electrical circuit. These protection devices are satisfactory when protecting devices and circuit components which themselves are capable of tolerating, at least for a short period of time, a circuit overload condition without appreciable injury, such as might be caused by the application of high voltage and attendant high current caused by a short circuit.
There are, however, circuits and devices that are incapable of tolerating even a momentary overload condition and, therefore, such protection devices and/or circuits are not satisfactory in preventing injury from occurring. There are many other installations where, although the protected devices or components can tolerate a bit of overvoltage, their life or subsequent performance is impaired to some extent when subjected to a momentary surge. That is, a surge may not cause their failure, but will degrade their vitality.
One situation that can cause an unexpected surge to occur is one involving electrical storage devices, such as capacitors and inductors, that can discharge or untimely cause a current or voltatge to occur back into a power supply network, thereby causing related devices connected to the power supply to experience overvoltage or overcurrent conditions even though the source or supply voltage remains within limits. A circuit which is suitable for providing some protection under such conditions is disclosed in FIG. 7 and related illustrations of copending Patent Application Ser. No. 654,926, filed Feb. 2, 1976, now abandoned in favor of continuing application Ser. No. 898,309, now U.S. Pat. No. 4,134,044 commonly assigned to the same assignee. A phototransistor controlled by the brightness of a light emitting diode connected across the dc output of the power supply determines the application of gate voltage to a triac. This conduction timing "regulates" the output even though there is no regulation of the ac input to the power supply. The circuit is protected against surges of ac input by the gating of another triac that operates a coil connected for opening its contacts in series with the source line.
Although the circuit does open the line in the presence of sustained overvoltage input conditions, its operation is somewhat hesitant when the voltage conditions are borderline. Further, the dc timing connection does provide regulation in the case of a dc voltage output above a predetermined danger level, but the action does not ensure that a sudden surge will result in opening the power circuit. That is, there is no absolute protection provided.
Patent Application Ser. No. 803,365, filed June 3, 1977 now U.S. Pat. No. 4,135,223, and also assigned to the same assignee, discloses a circuit for sensing an ac voltage overload condition using a bridge circuit for first converting the ac to dc and a pair of photodiode-phototransistors (optocouplers) for switching a gated semiconductor, and hence producing suitable triggers for actuating power switches.
Although such circuit has proven to be an improvement over previous circuits, the interposing of the bridge has introduced delays in sensing the overload before actual protection switching begins. Furthermore, the circuit has proven susceptible to stray voltages to an unsatisfactory degree. Gating isolation between polarities has not been altogether satisfactorily achieved and there has been insufficient isolation between the gate and main terminal signals on the gated semiconductor, thereby creating occasional false gating.
Therefore, it is a feature of the present invention to provide an improved electronic protection circuit operating with respect to sensing an ac voltage for overload condition without using either a bridge or an optocoupler.
It is another feature of the present invention to provide an improved electronic protection circuit utilizing a gated semiconductor operated in such a manner so as to isolate the gate and main terminals thereof.
It is still another feature of the present invention to provide an improved electronic protection circuit utilizing a voltage divider network for effectively sensing a voltage overload of a single polarity, while utilizing a slow-acting breakover device ahead of gating a semiconductor so as to prevent premature triggering because of stray noise.